Animate side panel opening and closing.

Create an animation that slides the side panel out from the side of the
browser. This updates the side panel and side panel coordinator to use
an enum, SidePanelContentState, to convey when a view should be hidden
and allow us to delay hiding it until the side panel has been closed.

Bug: 328295071
Change-Id: I40b099a1a3d0469ba815caf152fb0ecc3562af2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406892


Commit-Queue: Caroline Rising <corising@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280923}